Out of Date

This
website has not been updated for some years. This website has been left as it
may still contain useful content.

The Ecological Effects of Roads

By Reed Noss, PhD

The following article previously appeared two years ago in
"Killing Roads" under the name "Diamondback". It remains the definitive
summary of the effects of roads on biological diversity. An extensive
bibliography was prepared for the piece and is available from PAW NET (PAW
NET, 117 Main St., Brattleboro, VT 05301, 802-257-4878) for $3.00. It is an
indispensable tool for activists. [but see note at end]

Despite heightened recognition (by informed people) of the harmful effects
of roads, road density continues to increase in the US and other countries.
Federal, state, and local transportation departments devote huge budgets to
construction and upgrading of roads. Multinational lending institutions, such
as the World Bank, finance roads into pristine rainforest, which usher in a
flood of settlers who destroy both the rainforest and the indigenous cultures.
Public land-managing agencies build thousands of miles of roads each year to
support their resource extraction activities, at a net cost to the taxpayer.
The US Forest Service alone plans to build or reconstruct almost 600,000 miles
of roads in the next 50 years. Most public agencies disregard the ecological
impacts of roads, and attempt to justify timber roads as benefiting recreation
and wildlife management. Even when a land manager recognises the desirability
of closing roads, he or she usually contends that such closures would be
unacceptable to the public.

This article will review some ecological effects of roads, with emphasis on
impacts to wildlife (broadly defined). My concern is with all roads, from
primitive logging roads to four-lane highways. Although the effects of
different types of roads vary, virtually all are bad, and the net effect of
all roads is nothing short of catastrophic. The technical literature that
pertains to this topic is vast, and an entire book would be needed to
summarise it adequately. Consider this only an introduction, or an "executive
summary" of a massive tragedy.

Direct effects, such as flattened fauna, are easy to see. In contrast, many
indirect effects of roads are cumulative and involve changes in community
structure and ecological processes that are not well understood. Yet, these
long-term effects signal a deterioration in ecosystems that far surpasses in
importance the visual and olfactory insult to us of a bloated deer by the
roadside.

Direct Effects

Roadkills

The above statement notwithstanding, roadkill can have a significant impact
on wildlife populations. The Humane Society of the US and the Urban Wildlife
Research Centre have arrived at a conservative figure of one million animals
killed each day on highways in the United States. When I-75 was completed
through a major deer wintering area in northern Michigan, deer road mortality
increased by 500%. In Pennsylvania, 26,180 deer and 90 bears were killed by
vehicles in 1985. These statistics do not account for animals that crawl off
the road to die after being hit. Also, roadkill statistics are invariably
biased toward mammals, against reptiles, amphibians, and probably birds, and
do not include invertebrates at all (who wants to count the insects smashed on
windshields and grills?).

Vehicles on high-speed highways pose the greatest threat to wildlife.
Unpaved roads, particularly when "unimproved", are less dangerous. Roadkill
usually increases with volume of traffic. In one Texas study, however,
mortality was greatest on roads with intermediate volumes, presumably because
higher-volume roads had wider rights-of-way that allowed better visibility for
animals and drivers alike. Increases in traffic volume do result in more
collisions on any given road, and in our profligate society more people means
more cars on virtually every road.

Florida is a rapidly developing state with more than 1000 new human
residents each day and over 50 million tourists annually. Primary and
interstate highway mileage has increased by 4.6 miles per day for the last 50
years. Hence it is no surprise that roadkills are the leading known cause of
death for all large mammals except White-tailed Deer.

Roadkills of Florida Black Bear, a subspecies listed as threatened by the
state, have been rising sharply in recent years, from 2-3 per year in the
1970s to 44 in 1989. Many of the bears are killed on roads through public
lands, in particular the Ocala National Forest. Seventeen Florida Panthers,
one of the most endangered subspecies of mammals in the world, are known to
have been killed on roads since 1972. Since 1981, 65% of documented Florida
Panther deaths have been roadkills, and the population of only about twenty
individuals is unlikely to be able to sustain this pressure. An average of 41
Key Deer, a species listed as Endangered by the US Fish and Wildlife Service,
were killed on roads yearly from 1980 through 1986, and 57 were killed in
1987. Roadkill is also the leading cause of mortality for the American
Crocodile, also an Endangered species, in south Florida. The Florida Scrub
Jay, a Threatened species, has been found to suffer considerable mortality
from collision with vehicles, and researchers have concluded that these birds
cannot maintain stable populations along roads with considerable high-speed
traffic.

Snakes are particularly vulnerable to roadkill, as the warm asphalt
attracts them; yet their carcasses are seldom tallied. Herpetologists have
noted dramatic declines of snakes in Paynes Prairie State Preserve near
Gainesville, Florida, which is crossed by two four-lane highways. This
preserve was once legendary for its diversity and density of snakes, but no
more. Similarly, a study of south Florida herpetofauna by Wilson and Porras
attributed declines in many snakes to the increasing road traffic in that
region.

Roadkill is a classic death-trap phenomenon. Animals are attracted to roads
for a variety of reasons, often to their demise. Snakes and other ectotherms
go there to bask, some birds use roadside gravel to aid their digestion of
seeds, mammals go to eat de-icing salts, deer and other browsing herbivores
are attracted to the dense vegetation of roadside edge, rodents proliferate in
the artificial grasslands of road verges, and many large mammals find roads to
be efficient travelways. Songbirds come to dustbathe on dirt roads, where they
are vulnerable to vehicles as well as predators. Vultures, Crows, Coyotes,
Raccoons, and other scavengers seek out roadkills, often to become roadkills
themselves.

Road Aversion and other Behavioural Modifications

Not all animals are attracted to roads. Some have learned that roads bring
unpleasant things, such as people with guns. Species that show road aversion
exhibit decreasing densities toward roads. Various studies report that Turkey,
White-tailed Deer, Mule Deer, Elk, Mountain Lions, Grizzly Bear, and Black
Bear avoid roads. When these animals are disturbed by vehicles, they waste
valuable energy in flight. Other studies show conflicting results, which
usually can be explained by differences in road use. Certain bird species also
have been found to avoid roads, or the forest edges associated with roads. In
the Netherlands, researchers found some bird species to be displaced up to
2000 meters from busy highways.

The American Elk is one of the best-studied species with respect to road
aversion. Elk avoidance of roads is clearly a learned response (they do not
avoid natural edges), and is related to traffic volume and hunting pressure.
In western Montana, Jack Lyon found that Elk avoid areas within 1/4 - 1/2 mile
of roads, depending on traffic, road quality, and the density of cover near
the road. According to work by Jack Thomas in Oregon, a road density of one
mile per square mile of land results in a 25% reduction in habitat use by Elk;
two miles of road per square mile can cut Elk habitat use by half. As road
density increases to six miles of road per square mile, Elk and Mule Deer
habitat use falls to zero. Elk in some areas have learned that roads are
dangerous only in the hunting season, and do not show road aversion in other
seasons. Other studies suggest that Elk avoid open roads, but not closed
roads. Where hunting pressure is high however, even closed roads may be
avoided because so many hunters walk them.

Grizzly Bears also may be displaced by roads. In British Columbia,
Grizzlies were found to avoid areas within 1/2 mile of roads. A study in the
Cabinet Mountains of northwestern Montana determined that the mean distance of
Grizzly radio-telemetry signals from open roads (2467 m) was significantly
greater than the mean distance from closed roads (740 m). Other studies have
found that Grizzlies avoid areas near roads, especially by day, even when
preferred habitat and forage are located there. This is particularly alarming,
because in Yellowstone National Park, which has the second largest Grizzly
population in the lower 48, roads and developments are situated in the most
productive Grizzly Bear habitat Natural movements of Grizzly Bears may also be
deflected by roads, as Chuck Jonkel has documented in Montana. In other cases,
however, Grizzlies may use roads as travelways, particularly when they find
off-road travel difficult due to dense brush or logging slash. Grizzlies have
also learned to exploit the hastened growth of forage plants near roads in
spring. Similarly, the abundance of soft mast such as Pokeberry and Blackberry
along road edges attracts Appalachian Black Bears in summer. Any advantages
associated with roads for either bear species are outweighed by the increase
in sometimes fatal (usually for the bear, unfortunately) encounters with
humans.

Wild animals can become habituated to roads. Thirty years ago, for example,
bears in Yellowstone, the Great Smokies, and other parks often sat along the
roadsides and picnic areas waiting for handouts from tourists. When parks
disallowed handouts and relocated habituated hears, the attraction subsided.
In any area where animals are exposed to frequent human activity, habituation
can be expected. This is not necessarily a desirable response, however.
Although animals that are attracted to roads and vehicles do not waste energy
reserves in flight response, some of them become aggressive toward people.
Aggressive behaviour of habituated animals has been noted in bears, Mule Deer,
Elk, Bighorn Sheep, Bison, and other species. Conflicts occur most often when
humans approach animals closely in order to feed or photograph them. A few
years ago in the Smoky Mountains, a bear reportedly chomped on a baby's face
when a parent held it close for a kissing photo - the baby's cheek had been
smeared with honey. Such encounters usually result in relocation or killing of
the "problem" animals, though the real problem is human stupidity. Studies of
Grizzly Bears in Montana and British Columbia have found that bears habituated
to human activity especially moving vehicles, are more vulnerable to legal and
illegal shooting.

Fragmentation and Isolation of Populations

Some species of animals
simply refuse to cross barriers as wide as a road. For these species, a road
effectively cuts the population in half. A network of roads fragments the
population further. The remaining small populations are then vulnerable to all
the problems associated with rarity: genetic deterioration from inbreeding and
random drift in gene frequencies, environmental catastrophes, fluctuations in
habitat conditions, and demographic stochasticity (i.e., chance variation in
age and sex ratios). Thus, roads conrtribute to what many conservation
biologists consider the major threat to biological diversity: habitat
fragmentation. Such fragmentation may be especially ominous in the face of
rapid climate change. If organisms are prevented from migrating to track
shifting climatic conditions, and cannot adapt quickly enough because of
limited genetic variation, then extinction is inevitable.

In one of the first studies on habitat isolation by roads, D.J. Oxley and
co-workers in southeastern Ontario and Quebec found that small forest mammals
such as the Eastern Chipmunk, Grey Squirrel, and White-footed Mouse rarely
ventured onto road surfaces when the distance between forest margins (road
clearance) exceeded 20 meters. The authors suggested that divided highways
with a clearance of 90 meters or more maybe as effective barriers to the
dispersal of small mammals as water bodies twice as wide. Earlier work in
Africa had shown that tortoises, and young Ostrich, Wart hogs, and African
Elephants, had difficulty crossing roads with steep embankments. In Germany,
Mader found that several species of woodland carabid beetles and two species
of forest-dwelling mice rarely or never crossed two-lane roads. Even a small,
unpaved forest road closed to public traffic constituted a barrier. All of
these animals were physically capable of crossing roads but appeared to be
psychologically constrained from venturing into such openings. In Ontario,
Merriam and co-workers found that narrow gravel roads were "quantitative
barriers" to White-footed Mice in forest fragments; many fewer mice crossed
roads than moved an equal distance in the forest alongside roads.

expect that the barrier effect of roads would be less severe in more open
habitats, where the contrast between the road and adjoining habitat is less.
Yet, a study by Garland and Bradley of the effects of a four-lane highway on
rodents in the Mojave Desert found that rodents almost never crossed the road.
Of eight species captured, marked, and recaptured, only an adult male Antelope
Ground Squirrel crossed the entire highway. No roadkills were observed,
suggesting that few rodents ever ventured onto the highway.

Animals far more mobile than rodents and beetles may hesitate to cross
roads. In the Southern Appalachians, Brody and Pelton found that
radlo-collared Black Bears almost never crossed an interstate highway. In
general, the frequency at which bears crossed roads varied inversely with
traffic volume. Bears appeared to react to increasing road densities by
shifting their home ranges to areas of lower road density. The power of flight
may not override the barrier effect of roads for some bird species. Many
tropical forest birds are known to be averse to crossing water gaps no wider
than a highway. Further research is needed to determine if these species react
to road clearings as they do to water gaps.

Thus, populations of many animal species divided by a heavily travelled
road may be just as isolated from one another as if they were separated by
many miles of barren urban or agricultural land. Larry Harris and Peter
Gallagher, writing in a recent Defenders of Wildlife publication on habitat
corridors ("Preserving Communities & Corridors" available from Defenders,
1244 19th St. NW, Washington, DC 20036; $10 each), put the road fragmentation
problem into proper perspective:

"Consider this triple jeopardy: At the same time that development reduces the
total amount of habitat, squeezing remaining wildlife into smaller and more
isolated patches, the high-speed traffic of larger and wider highways
eliminates more and more of the remaining populations."

To the extent that various plant species depend on road-averse
animals for dispersal, roads fragment plant populations as well.

Pollution

Pollution from roads begins with construction. An immediate
impact is noise from construction equipment, and noise remains a problem along
highways with heavy traffic. Animals respond to noise pollution by altering
activity patterns, and with an increase in heart rate and production of stress
hormones. Sometimes animals become habituated to increased noise levels, and
apparently resume normal activity. But birds and other wildlife that
communicate by auditory signals may be at a disadvantage near roads. Highway
noise can also disrupt territory establishment and defence. A study by Andrew
Barrass found that toads and tree frogs showed abnormal reproductive behaviour
in response to highway noise.

Vehicles emit a variety of pollutants, including heavy metals, carbon
dioxide, and carbon monoxide, all of which may have serious cumulative
effects. Combustion of gasoline containing tetraethyl lead, and wear of tires
containing lead oxide, result in lead contamination of roadsides. Although
unleaded gasoline now accounts for more than half of all gasoline used in the
US, lead persists in soils and the food web for long periods. In Kansas, lead
levels in roadside soils and vegetation in the early 1980s were two to three
times greater than from near roads with similar traffic volumes in 1973 and
1974, when the use of unleaded gasoline was 42% lower.

Many studies have documented increasing levels of lead in plants with
proximity to roads, and with increases in traffic volume. Plant roots take up
lead from the soil, and leaves take it up from contaminated air or from
particulate matter on the leaf surface. This lead moves up the food chain,
with sometimes severe toxic effects on animals, including reproductive
impairment, renal abnormalities, and increased mortality rates. Food chain
effects can switch between aquatic and terrestrial pathways. Lead
concentrations in tadpoles living near highways can be high enough to cause
physiological and reproductive impairment in birds and mammals that prey on
tadpoles. Less is known about the effects of other heavy metals, such as zinc,
cadmium, and nickel. Motor oil and tires contain zinc and cadmium; motor oil
and gasoline contain nickel. These metals, like lead, have been found to
increase with proximity to roads, and with increasing traffic volume and
decreasing soil depth. Earthworms have been found to accumulate all these
metals, in concentrations high enough to kill earthworm-eating animals. These
roadside contaminants can be carried far from roads by wind and water. lead
contamination has been noted up to 100 miles from the nearest metropolitan
area.

The maintenance of roads and roadsides also introduces a variety of
pollutants into roadside ecosystems. Americans like their roads free of ice
and dust, and their roadsides free of weeds. The effects of herbicides on
wildlife and ecosystems have been poorly studied, but anyone who has witnessed
the destruction of wild flowers and other plants along roadsides (even through
parks) for the sake of tidiness has cause to complain.

Highway de-icing programs are notorious sources of saline pollution. In the
early 1970s, it was estimated that 9-10 million tons of sodium chloride, 11
million tons of abrasives, and 30,000 tons of calcium chloride were used in
the US each year for highway de-icing. As noted above, many animals are
attracted to this salt and end up as roadkills or at least get a dose of the
salt's toxic additives, including cyanide compounds. Drainage of salt-laden
water from roads into aquatic ecosystems may stimulate growth of blue-green
algae; the chloride concentration of major water bodies near urban areas has
been found to increase by as much as 500%. Furthermore, sodium and calcium ion
exchange with mercury releases toxic mercury into these Systems. The cyanide
ions from rust-inhibiting additives are extremely toxic to fish.

In many rural areas, waste oil from crankcases is sprayed onto unpaved
roads for dust control. A 1974 study estimated that some 100 million gallons
of waste oil are sprayed on dirt roads in the US each year. Only about 1% of
this oil remains in the top inch of a road surface. Much of it reaches water
bodies, where it coats the surface, limiting oxygen exchange and sunlight
penetration and having toxic effects on aquatic organisms.

Impacts on Terrestrial Habitats

The impacts of roads on terrestrial
ecosystems include direct habitat loss; facilitated invasion of weeds, pests,
and pathogens, many of which are exotic (alien); and a variety of edge
effects. Roads themselves essentially preempt wildlife habitat. A 1974 report
by the Council on Environmental Quality estimated that one mile of interstate
highway consumes up to 48 acres of habitat. Logging roads result in the
clearing of about 50 acres for each square mile of commercial forest (i.e., 10
acres are deforested for every mile of road, and each square mile of forest
averages 5 miles of road). Road construction also kills animals and plants
directly, and may limit long-term site productivity of roadsides by exposing
low nutrient subsoils, reducing soil water holding capacity, and compacting
surface materials. It also makes slopes more vulnerable to landslides and
erosion, which in turn remove additional terrestrial wildlife habitat and
degrade aquatic habitats.

Some species thrive on roadsides, but most of these are weedy species. In
the Great Basin, rabbit brush is usually more abundant and vigorous along
hard-surfaced roads than anywhere else, because it takes advantage of the
runoff water channelled to the shoulders, Although certainly attractive, the
common rabbit brush species are in no danger of decline, as they invade
disturbed areas such as abandoned farmsteads and fence rows, and are
considered an indicator of overgrazing. In the Mojave Desert, Creosote Bush is
another abundant species that opportunistically exploits the increased
moisture levels along roadsides.

Many of the weedy plants that dominate and disperse along roadsides are
exotics. In some cases, these species spread from roadsides into adjacent
native communities. In much of the west, Spotted Knapweed has become a serious
agricultural pest. This Eurasian weed invades native communities from
roadsides, as does the noxious Tansy Ragwort. In Florida, a state plagued by
exotic plants, one of the biggest offenders is Brazilian Pepper. This tall,
fast-growing shrub readily colonises roadside habitats. When soil in adjacent
native habitats is disturbed by off-road vehicles, Brazilian Pepper invades.
Invasion by Brazilian Pepper and other roadside exotics is becoming a serious
problem in the Atlantic coastal scrubs of south Florida, communities endemic
to Florida and containing many rare species. Another invasive exotic,
Melaleuca, is expanding from roadsides and dominating south Florida wetlands.
In southwest Oregon and northwest California, an apparently introduced
root-rot fungus is spreading from logging roads and eliminating populations of
the endemic Port Orford Cedar.

Opportunistic animal species also may benefit from roads. Grassland
rodents, for example, sometimes extend their ranges by dispersing along
highway verges. In 1941, L.M. Huey documented a range extension of pocket
gophers along a new road in the arid Southwest. Meadow voles have been found
to colonise new areas by dispersing along the grassy rights-of-way (ROWs) of
interstate highways. Roads also facilitate dispersal of prairie dogs. In 1983,
Adams and Geis reported that more species of rodents may be found in highway
ROWs than in adjacent habitats, though several species avoid ROW habitat.
Birds associated with grassland or edge habitat, such as the European
Starling, Brewer's and Red-winged Blackbirds, Brown-headed Cowbird, Indigo
Bunting, White-throated Sparrow, Song Sparrow, and Killdeer, all have been
found to increase in abundance near roads. Cliff and Barn Swallows, Starlings,
House Sparrows, and Rock Doves (the latter three are exotic species in North
America) often nest and roost in highway bridges. Many species of birds and
mammals feed on roadkill carrion.

Some people claim that increases in grassland, edge, and other
opportunistic species near roads constitute a benefit of roads. But increased
density near roads may not be favourable for the animals involved, if the road
exposes them to higher mortality from heavy metal poisoning or collision with
vehicles. In this sense, a road can be an "ecological trap" and a "mortality
sink" for animal populations. Furthermore, the species that may benefit from
roads are primarily those that tolerate or even thrive on human disturbance of
natural landscapes, and therefore do not need attention from conservationists
(except occasional control). Many of these weedy species are exotic, and have
detrimental effects on native species.

Edge effects, once considered favourable for wildlife because many game
species (e.g., White-tailed Deer, Eastern Cottontail, Northern Bobwhite) are
edge-adapted, are now seen as one of the most harmful consequences of habitat
fragmentation. Especially when it cuts through an intact forest, a road
introduces a long swath of edge habitat. Forest edge is not a line, but rather
a zone of influence that varies in width depending on what is measured.
Changes in microclimate, increased blowdowns, and other impacts on vegetation
may extend 2-3 tree-heights into a closed-canopy forest. Shade-intolerant
plants, many of them exotic weeds, colonise the edge and gradually invade
openings in the forest interior. Dan Janzen found weedy plant species invading
treefall gaps in a Costa Rican forest up to 5 kilometers from the forest edge.
Changes in vegetation structure and composition from edge effects can be more
persistent than effects of clearcutting, from which at least some forest types
will eventually recover, if left alone.

The Brown-headed Cowbird, originally abundant in the Great Plains but now
throughout most of North America because of forest fragmentation, is known to
penetrate forests at least 200 meters from edge. The cowbird is a brood
parasite that lays its eggs in the nests of other bird species and can
significantly reduce the reproductive success of its hosts. Forest birds, most
of which did not evolve with the cowbird and are not well adapted to its
parasitism, may show serious declines in areas where cowbirds have become
common. In addition, many opportunistic nest predators, such as jays, crows,
Raccoons, and Opossums, are common in roadside environments (partially because
of supplemental food in the form of carrion) and often concentrate their
predatory activities near edges. Increases in nest predation from these
opportunists can extend up to 600 meters from an edge) as shown by David
Wilcove using artificial nest experiments.

A narrow logging road with no maintained verge would not be expected to
generate substantial edge effects, particularly if surrounded by a tall forest
canopy. In this sense, the road would not differ much from hiking trail (even
trails create some edge effects, however, such as invasion of weedy plants
caused by pant-legs dispersal). As forest roads are "improved," road clearance
increases and allows more penetration of sunlight and wind. Edge species are
then attracted to these openings. Two-lane roads with maintained rights-of-way
and all interstate highways are lined by edge habitat. A forest criss-crossed
by improved roads may be largely edge habitat, and its value for conservation
of native flora and fauna diminished accordingly.

Impacts on Hydrology and Aquatic Habitats

Road construction alters the
hydrology of watersheds through changes in water quantity and quality, stream
channel morphology, and ground water levels. Paved roads increase the amount
of impervious surface in a watershed, resulting in substantial increases in
peak runoff and storm discharges. That usually means flooding downstream.
Reduced evapo-transpiration within road rights-of-way may also result in
increased runoff and streamflows. However, increases in streamflows in
forested watersheds are not usually significant unless 15% or more of the
forest cover is removed by road construction and associated activities such as
logging. When a road bed is raised above the surrounding land surface, as is
normally the case, it will act as a dam and alter surface sheet flow patterns,
restricting the amount of water reaching downstream areas. Mike Duever and
co-workers found this to be a significant problem in the Big Cypress
Everglades ecosystem of South Florida. Ditches dug for road drainage often
drain adjacent wetlands as well. The US Fish and Wildlife Service, in 1962,
estimated that 99,292 acres of wetlands in western Minnesota had been drained
as a result of highway construction. This drainage occurred at a rate of 2.33,
2.62, and 4.10 acres of wetland per mile of road for state and federal,
county, and township highways, respectively.

Roads concentrate surface water flows, which in turn increases erosion.
Megahan and Kidd, in 1972, found that erosion from logging roads in Idaho was
220 times greater than erosion from undisturbed sites. Logging roads used by
more than 16 trucks per day may produce 130 times more sediment than do roads
used only by passenger cars. Incision of a slope by roadcuts in mountainous
areas may intercept subsurface flow zones, converting subsurface flow to
surface flow and increasing streamflow rates. Water tables are almost always
lowered in the vicinity of a road.

The impacts of roads on fish and fisheries have long concerned biologists.
Increased erosion of terrestrial surfaces almost inevitably results in
increased sedimentation of streams and other water bodies. Even the best
designed roads produce sediment, and unpaved roads continue to produce
sediment for as long as they remain unvegetated. A divided highway requiring
exposure of 10 to 35 acres per mile during construction produces as much as
3000 tons of sediment per mile. In a study of the Scott Run Basin in Virginia,
Guy and Ferguson found that highway construction contributed 85% of the
sediment within the basin. The yield was 10 times that normally expected from
cultivated land, 200 times that from grasslands, and 2000 times that from
forest land. Studies in northwestern California show that about 40% of total
sediment is derived from roads and 60% from logged areas. Much of the
sedimentation associated with roads occurs during mass movements (i.e.,
landslides) rather than chronic surface erosion. Roads dramatically increase
the frequency of landslides and debris flows. Studies in Oregon have found
that roads trigger up to 130 times more debris torrents than intact
forest.

Increased sediment loads in streams have been implicated in fish declines
in many areas. A 1959 study on a Montana stream, reported by Leedy in 1975,
found a 94% reduction in numbers and weight in large game fish due to
sedimentation from roads. Salmonids are especially vulnerable to sedimentation
because they lay their eggs in gravel and small rubble with water flow
sufficient to maintain oxygen supply. Fine sediments may cement spawning
gravels, impeding the construction of redds. Increases in fine sediments also
reduce the availability of oxygen to eggs and increase embryo mortality.
Stowell and co-workers reported that deposition of 25% fine sediments in
spawning rubble or gravel reduces fry emergence by 50%. Sedimentation also has
negative effects on the invertebrate food supply of many fish. Furthermore,
destruction of riparian vegetation by road construction results in higher
water temperatures, which reduces dissolved oxygen concentrations and
increases fish oxygen demands (a "double whammy"). If the fishing public was
adequately informed of the negative effects of roads on fisheries, perhaps all
but the laziest would demand that most roads on public lands be closed and
revegetated!

Indirect Effects

Access

The most insidious of all effects of roads is the access they
provide to humans and their tools of destruction. Let's face it, the vast
majority of humans do not know how to behave in natural environments. Fearful
of experiencing Nature on its own terms, they bring along their chainsaws,
ATVs, guns, dogs and ghetto blasters. They harrass virtually every creature
they meet, and leave their mark on every place they visit. The more
inaccessible we can keep our remaining wild areas to these cretins, the safer
and healthier these areas will be. Those humans who respect the land are
willing to walk long distances. If this is an "elitist" attitude, so be it;
the health of the land demands restrictions on human access and behaviour.

Many animal species decline with increasing road density precisely because
roads bring humans with guns. For many large mammals, road aversion is not
related to any intrinsic qualities of the road, but rather to their learned
association of roads with danger. In other Cases, mammals may continue to use
roads because they provide convenient travelways or food supply, but are
unable to maintain populations where road densities are high because of the
mortality they suffer from legal or illegal hunting, or roadkill.

An historical study by Richard Thiel in northern Wisconsin, supplemented by
modern radio-telemetry, showed that road density was the best predictor of
Grey Wolf habitat suitability. As road density increased in the study area,
the Wolf population declined. Wolves failed to survive when road densities
exceeded .93 mile per square mile (.58 km per square km). Similar studies in
Michigan and Ontario by Jensen and co-workers, and in Minnesota by Mech and
co-workers, found a virtually identical threshold level for the occurrence of
Wolves. Roads themselves do not deter Wolves. In fact, Wolves often use roads
for easy travel or to prey on the edge-adapted White-tailed Deer. But roads
provide access to people who shoot, snare, trap, or otherwise harass wolves.
David Mech found that over half of all known Wolf mortality was caused by
humans, despite the "protection" of the Endangered Species Act.

Many other large mammal species have been found to decline with increasing
road access. The Florida Panther once ranged throughout the Southeast, from
South Carolina through southern Tennessee into Arkansas, Louisiana and extreme
eastern Texas. It is now restricted to south Florida, an area of poor deer and
Panther habitat, but the last large roadless area available in its range.
Problems associated with roads - roadkill, development, and illegal shooting -
are now driving it to extinction. A population viability analysis has
determined an 85% probability of extinction in 25 years, and a mean time to
extinction of 20 years. Proposed management interventions still yield 75% to
99% probabilities of extinction within 100 years.

Recently, Seminole Chief James Billie shot a Panther with a shotgun from
his pickup truck in the Big Cypress Swamp, ate it, and claimed this murder was
a native religious ritual. Billie eventually won his case, not on religious
grounds, but because taxonomists could not prove beyond all reasonable doubt
that the skull found in Billie's possession was that of a Florida Panther,
Felis concolor subspecies coryi (the various subspecies of Cougar differ
little from one another in morphology).

Biologists agree that the only hope for the Panther is reestablishment of
populations elsewhere within its historic range. But is there anywhere with
low enough road density to be safe? The best opportunity seems to be the 1.2
million acres in and around Okefenokee National Wildlife Refuge in southern
Georgia and Osceola National Forest in north Florida, recently connected by
purchase of Pinhook Swamp and its transfer to the Forest Service.
Experimenters testing the feasibility of Panther reintroduction in this area
released 5 neutered and radio-collared Texas Cougars, a subspecies closely
related to F.c. coryi, into this habitat. Within a month, one cat died of
unknown causes. Two more cats were killed by hunts soon thereafter. The final
two cats discovered livestock (a goat pasture and an exotic game reserve), and
were removed from the wild. This setback in the Panther reintroduction program
demonstrates that even one of the wildest areas in the Southeast is still far
too human-accessible for Panthers to survive. Except for the wettest part of
the Okefenokee Swamp, the poorest Panther habitat, the area is riddled with
roads and swarming with gun-toting "Crackers" and their hounds.

Other large mammals that suffer from road access include Cougars (western
version of Fc.) and Grizzly Bears. A radio-telemetry study in Arizona and
Utah, by Van Dyke and co-workers, found that Cougars avoided roads (especially
paved and improved dirt roads) whenever possible, and established home ranges
in areas with the lowest road densities. In southeastern British Columbia,
McLellan and Mace found that a disproportionate amount of Grizzly Bear
mortality occurred near roads. Of 11 known deaths, 7 bears were definitely
shot and another 3 were probably shot from roads. Dood and co-workers found
that 32% of all hunting mortality and 48% of all non-hunting mortality of
Grizzlies in Montana occurred within one mile of a road. Knick and Kasworm
recently found that illegal shooting was the primary cause of death for
Grizzlies in the Selkirk and Cabinet-Yaak ecosystems, and concluded that the
ability of regions to maintain viable populations of Grizzly Bears is related
to road density and human access.

Road access imperils Black Bears, too. In the Southern Appalachians, Mike
Pelton has estimated that bears cannot maintain viable populations when road
density exceeds .8 miles of road per square mile. Later studies found that the
situation is more complicated, and is related to traffic volume and other road
use factors. The primary effect of roads on bears in the Southern Appalachians
is to expose them to increased hunting. Hunting with the aid of trained hounds
is the major source of mortality for bears in this region, including within
National Parks and other sanctuaries, and is encouraged by the trade in bear
gall bladders to the Oriental market.

The problem of road access and overhunting is often attributed to
inadequacies of human ethics and law enforcement, rather than to any effect of
the roads themselves. But as Richard Thiel pointed out, in discussing the Grey
Wolf in northern Wisconsin, "Ultimately, the survival of wolves will depend on
a change in human attitudes. Until then road densities are important in
determining whether an area can sustain a viable population of wolves." We may
have to wait a long time before attitudes toward Nature improve, but roads can
be closed today.

Other consequences of road access include over collecting of rare plants
(e.g., cacti, orchids, and ginseng) and animals (e.g., snakes for the pet
trade), the removal of snags near roadsides by firewood cutters, and increased
frequency of fire ignitions. Removal of snags eliminates habitat for the many
cavity-nesting and roosting birds and mammals. In the Blue Mountains of
eastern Oregon and Washington, for example, 39 bird and 23 mammal species use
snags for nesting or shelter. Woodpeckers are among the cavity-nesting birds
known to be critically important in dampening forest insect outbreak. Thus,
snag removal along roadsides is an anthropogenic edge effect that may have
far-reaching effects on entire ecosystems.

Humans are suspected to cause at least 90% of wildfires in the US, over
half of which begin along roads. In 1941, Shaw and co-workers reported 78% of
all anthropogenic fires occurred within 265 feet of a road. In New Jersey, the
origins of 75% of all forest fires were traced to roadsides. Although fire is
a natural process with beneficial effects on many ecosystems, natural fires
and anthropogenic fires differ in many ways. One important difference is
frequency; anthropogenic fires may occur more frequently than the natural fire
return interval for a given ecosystem type. Another important difference is
seasonality. In Florida, for example, most anthropogenic fires occur in
winter, whereas natural lightning fires occur in late spring and summer.
Research in longleaf Pine-Wiregrass communities, which under natural
conditions experience low-intensity ground fires at 2 to 5 year intervals, has
determined that summer fires promote higher herbaceous plant diversity and
flowering. Winter fires caused by humans tend to promote monotonous,
shrub-dominated (eg. saw palmetto) communities. It is a curious contradiction
that the US forest service often justifies high road densities as necessary to
provide fire control, when in fact most fires begin along roads.

Of the disturbances promoted by road access, perhaps the most devastating
is development. Highways introduce pressures for commercial development of
nearby land. Highway interchanges inevitably become nodes of ugly
commercialism. Arterial streets encourage commercial strip development, and
new rural and suburban roads bring in commercial, industrial, and residential
development. Internationally funded road-building in Third World countries
introduces hordes of immigrants, who quickly cut and burn the native forest.
In Brazilian Amazonia, Philip Fearnside reported that road development funded
by the World Bank facilitates the entry of settlers whose land claims
(established by clearing the forest) justify building more roads. Thus, roads
and deforestation interact in a positive feedback relationship. Roads bring
settlement and development, which in turn call for more roads.

Cumulative Effects

So far, this article has discussed effects of roads
mostly in isolation from one another. Indeed, almost all research on road
problems has looked at one factor at a time, be it lead pollution, roadkill,
edge effects, or access. In real ecosystems, however, these factors interact
in complex ways, with long-term effects at several levels of biological
organisation.

To illustrate the complexity of possible impacts, consider this scenario: A
network of roads is built into prime Grey Wolf habitat in northern hardwoods
forest. Hunters flock into the area, depressing the Wolf population. Some
Wolves are killed by vehicles. Eventually, the Wolf becomes extinct in this
region. In the absence of Wolf predation, and with the abundance of brushy
roadside edge habitat, the White-tailed Deer population explodes. Fires
started by humans along roadsides create even more deer habitat. Hunters and
vehicles take some deer, but they cannot keep up. The burgeoning deer
population overbrowses the forest eliminating regeneration of favoured Eastern
Hemlock, Arbor Vitae, Canada Yew, and a number of rare herbaceous plants. As a
result, the floristic composition and vegetation structure of the forest
gradually change. With reduced understory density due to heavy browsing, many
warblers and other forest songbirds undergo serious declines. With Wolves
gone, opportunistic medium-sized mammals ("mesopredators") such as Opossums
and Raccoons increase in abundance and feed on the eggs and nestlings of
songbirds, many of which nest on or near the ground, further depressing their
numbers. Brown-headed Cowbirds parasitise these beleaguered songbirds within
200 meters or so of road edges. Cutting of snags for firewood along the
roadsides decimates cavity-nesting bird populations. Populations of insect
pests now cycle with greater amplitude, resulting in massive defoliation. The
roads also bring in developers, who create new residential complexes, and
still more roads. Roadside pollutants from increased traffic levels poison the
food chain. The original forest ecosystem has been irretrievably
destroyed.

This scenario is fictitious, but every part of it has been documented
somewhere. Because many of the animal species most sensitive to roads are
large predators, we can expect a cascade of secondary extinctions when these
species are eliminated or greatly reduced. Recent research confirms that top
predators are often "keystone species", upon which the diversity of a large
part of the community depends. When top predators are eliminated, such as
through roadkill or because of increased access to hunters, opportunistic
mesopredators increase in abundance, leading to declines of many songbirds and
ground-dwelling reptiles and amphibians. In the tropics, predator removal can
lead to an increased abundance of mammals that eat large-seeded plants, which
in turn may result in changes in plant community composition and diversity
(see John Terborgh's article, "The Big Things that Run the World", reprinted
in Earth First!, 8-89).

Other keystone species may be similarly vulnerable to roads. The Gopher
Tortoise of the southeastern US, for example, digs burrows up to 30 feet long
and 15 feet deep. By a recent count, 362 species of commensal invertebrates
and vertebrates have been found in its burrows, and many of them can live
nowhere else. Yet, the slow-moving Gopher Tortoise is extremely vulnerable to
roadkill on the busy highways of this high growth region. Roads also provide
access to developers and poachers, the tortoise's biggest enemies. But the
effects of roads on Gopher Tortoises can be more subtle. Good Gopher Tortoise
habitat is longleaf Pine-Wiregrass, which requires frequent summer fires to
maintain its open structure. Although, as discussed above, many fires are
ignited along roadsides, the net effect of roads on this habitat has been to
stop the spread of fires that once covered areas the size of several counties.
Those roadside fires that do ignite are mostly winter burns, which are less
effective in controlling shrub invasion. As shrubs, oaks, and other hardwoods
overtake this ecosystem, they shade out the herbaceous plants upon which the
herbivorous Gopher Tortoise depends.

The net, cumulative effect of roads is to diminish the native diversity of
ecosystems everywhere. Habitats in many different places around the would are
invaded by virtually the same set of cosmopolitan weeds. Regions gradually are
homogenised - they lose their "character". Every place of similar climate
begins to look the same and most ecosystems are incomplete and missing the
apex of the food chain. The end result is an impoverishment of global
biodiversity.

What Can Be Done?

Mitigation

The traditional response of public agencies to road -
wildlife conflicts, in those rare instances when they do respond, is
"mitigation," i.e., build the road but design it so as to minimise its
impacts. For example, barren roadsides can be planted and stabilised by wire
netting in order to reduce erosion, landslides, and sedimentation of streams.
Stream culverts can be designed to minimise disruption of flow and bed
morphology. New roads can be located, and existing roads relocated, outside of
critical wildlife habitats (such as moist meadows, shrub fields, riparian
zones, and other Grizzly Bear feeding areas). Speed bumps and warning signs
can be installed to slow down motorists and reduce roadkill. Reflective
mirrors along roadsides and hood-mounted ultrasonic whistles are devices
intended to warn animals of approaching death-machines, but are still of
unproven benefit.

Road rights-of-way can be managed to maximise their potential as native
wildlife habitat and dispersal corridors. If wide swaths of old-growth
longleaf Pines are maintained along highway ROWs in the Southeast for example,
they may serve to connect isolated Red-cockaded Woodpecker populations. Such
corridors were recommended by a committee of the American Ornithologists'
Union. Some evdence suggests that Red-cockaded Woodpeckers may indeed disperse
along such corridors, but not across long expanses of unsuitable habitat. The
management of"roadside verges" for fauna and flora has a long history in
Britain, as reviewed by J.M.Way in 1977.

Undoubtedly, mitigation measures, if implemented intelligently, can reduce
the harmful effects of roads on wildlife. A 1982 report by Leedy and Adams,
for the US Department of Transportation and Fish and Wildlife Service,
summarises a variety of design and construction options to mitigate the
effects of roads. For reducing roadkiIIs, a combination of fencing and
underpasses has proven effective in many instances. Tunnels under roads were
used as early as 1958 in the United Kingdom to reduce roadkill of badgers and
have been used in several countries to reduce roadkill of amphibians (many
frogs toads, and salamanders migrate to their breeding ponds on wet spring
nights). Toad tunnels were constructed as early as 1969 in Switzerland, and
have been built throughout much of the United Kingdom, West Germany, the
Netherlands, and other countries under the auspices of the Fauna and Flora
Preservation Society and Herpetofauna Consultants international. A private
firm, ACO Polymer products Limited even specialises in the design and
production of amphibian tunnel and fencing systems (see Defenders 10-89).

In Colorado, underpasses and deer-proof fencing were constructed on I-70,
to channel movement of Mule Deer along a major migratory route, and have
proved fairly successful. D.F. Reed and co-workers, however, found that many
individual deer were reluctant to use a narrow underpass (3 meters wide and
high, and 30 meters long), and recommended that underpasses be significantly
wider. Biologists in various Western states are experimenting with one-way
gates that keep most deer off the highway but allow deer that get into the
highway ROW to escape. in southeastern Austialia, Mansergh and Scott
constructed a funnel-shaped rocky corridor and two tunnels of .9 X 1.2 meters
each beneath a road that bisected the breeding area of the rare Mountain
Pygmy-possum (the only marsupial hibernator known). The design proved very
successful in restoring natural movement and breeding behaviour of the
Pygmy-possums. One of the more controversial applications of the underpass
strategy has been in south Florida, for the sake of the Florida Panther. As
noted above, roadkill is the leading known cause of death for this subspecies.
Thus, when an extension of I-75 through the Everglades-Big Cypress Swamp was
proposed, conservationists reacted with alarm. When assured by highway and
wildlife officials that the new interstate would include fences and underpass
for Panthers, making it much less dangerous than the infamous Panther-smashing
Alligator Alley which it would replace many conservationists (including the
Florida Audubon Society and the Sierra Club) came out in support of the new
road.

How effective will these underpasses be in allowing for movement of
Panthers and other wildlife? Eighty-four bridges are being constructed on the
49 miles of new I-75 in Collier county, 46 of them designed solely for
wildlife movement. Each of these "wildlife crossings" consists of three
40-foot spans, for a total length of 120 feet with 8 feet of vertical
clearance. Much of the 120 feet will be under water, however, at least in the
wet season. There is no guarantee that these crossings will be functional for
Panthers and other large mammals. Even Thomas Barry, the project manager for
the Florida Department of Transportation, admits that the ideal solution would
have been to build a viaduct (elevated highway) across the entire stretch, but
that this solution was deemed too expensive. As advocated by Florida Earth
First!, the "ideal solution" would be to close Alligator Alley and all other
roads in the Everglades - Big Cypress bioregion, and to allow no new roads.
The desirability of this solution became more evident when we learned that the
new I-75 will include recreational access sites for ORVs, as recommended by
the Florida Game and Fresh Water Fish Commission.

The Preferred Alternative

In evaluating various mitigation options for
road-wildlife problems, it must be remembered that each is a compromise,
addresses only a subset of the multiple ecological impacts of roads, and is
far less satisfactory than outright road closure and obliteration. The serious
conservationist recognises that mitigation options should be applied only to
roads already constructed, and which will be difficult to close in the near
future (i.e., major highways). In such cases, construction of viaducts over
important wildlife movement corridors (as documented by roadkills) and other
critical natural areas should be vigorously pursued. Amphibian tunnels and
other smaller underpasses also should be constructed where needed. But the
bottom line is that no new roads should be built, and most existing roads -
especially on public lands - should be closed and obliterated. This is the
preferred alternative!

A priority system for determining which roads should be closed first is
necessary to guide conservation actions toward the most deserving targets. The
Grizzly Bear Compendium (Lefranc et al. 1987, pp.145-46) specifies which kinds
of roads should be closed on public lands to protect Grizzlies: Access roads
should be closed after harvesting and restocking, temporary roads and landings
should be obliterated, collector roads and loop roads should be closed in most
instances, local roads should be closed within one season after use, and
seismic trails and roads should be closed after operations have ceased. Bear
biologist Chuck Jonkel has long recommended an aggressive road closure program
on public lands. Public education on the rationale for closures, and strong
law enforcement, must accompany road closure programs if they are to be
effective. The Grizzly bear Compendium recommends that road use restrictions,
such as seasonal closures of roads in areas used only seasonally by bears, be
placed on roads that cannot be permanently closed.

In a series of publications, I have recommended that large core areas of
public lands be managed as roadless "wilderness recovery areas" (a concept
attributable to Dave Foreman). Buffer zones surrounding these core areas would
have limited access for recreation and other multiple-use" activities
consistent with preservation of the core preserves. Buffer zones also would
insulate the core areas from the intensive uses of the humanised landscape.
These large preserve complexes would be connected by broad corridors of
natural habitat to form a regional network.

As Keith Hammer has documented, however, road closures that appear on paper
may not function as such on the ground. Keith found that 38% of the putative
road closures on the Flathead National Forest in Montana would not bar
passenger vehicles. The road miles behind the ineffective barriers represented
44% of the roads reported by the Forest Service as being closed to all
motorised vehicles year-round. Gates, earthen berms, and other structures are
not usually effective in restricting road use. This is especially true in more
open-structured habitats, such as Longleaf Pine and Ponderosa Pine forests,
where motorists can easily drive around barriers. It may be that the only
effective road closures are those where the road is "ripped" and
revegetated.

The Forest Service and other public agencies will claim that road closures,
revegetation, and other restorative measures are too expensive to be
implemented on a broad scale. But much of the approximately $400 million of
taxpayers' money squandered annually by the Forest Service on below-cost
timber sales goes to road-building. Road maintenance is also expensive.
Virtually all of this money could be channelled into road closures and
associated habitat restoration. This work would be labor-intensive, and
providing income to the many laid off loggers, timber sale planners, and road
engineers - for noble jobs, rather than jobs of destruction! likewise, the
huge budgets of federal, state, and county highway departments could be
directed to road closures and revegetation, as well as viaducts and
underpasses to minimise roadkill on roads kept open.

We cannot expect our public agencies to shift to a more enlightened roads
policy without a fight. A lot of people make a lot of money designing and
building roads, and exploiting the resources to which roads lead. Nor can we
expect the slothful, ignorant populace to give up what they see as the
benefits of roads (fast transportation, easy access to recreational areas,
scenery without a sweat, etc) for the sake of bears and toads. Education of
the public, the politicians, and our fellow environmentalists about the
multiple and far-reaching impacts of roads is critical. As Aldo Leopold noted,
"recreational development is a job not of building roads into lovely country,
but of building receptivity into the still unlovely human mind" The greatest
near-term need is direct action in defence of existing roadless areas, and to
close roads where they are causing the most problems for native
biodiversity.

Dr. Noss is a member of the
Board of Governors of the Society for Conservation Biology. He is also the
science editor of Wild Earth and advisor to PAW NET and the Wildlands Project.
Nothing is worse for sensitive wildlife than a road. Over the last few
decades, studies in a variety of terrestrial and aquatic ecosystems have
demonstrated that many of the most pervasive threats to biological diversity -
habitat destruction and fragmentation, edge effects, exotic species invasions,
pollution, and overhunting - are aggravated by roads. Roads have been
implicated as mortality sinks for animals ranging from snake to wolves, as
displacement factors affecting animal distribution and movement patterns, as
population fragmenting factors, as sources of sediments that clog streams and
destroy fisheries, as sources of deleterious edge effects, and as access
corridors that encourage development, logging and poaching of rare plants and
animals. Road-building in National Forests and other public lands threatens
the existence of de facto wilderness and species that depend on
wilderness.

update 2002: Unable to find the original biblio--the address and phone
number don't work. There is a revised and updated version at Wildlands
Center for Preventing Roads along with a huge amount of reference
material.